Holographic optical trapping uses computer-generated holograms to trap and organize micrometer-scale objects into arbitrary three-dimensional configurations. No complementary method has been available in the prior art for examining optically trapped structures except for conventional two-dimensional microscopy. Three-dimensional imaging would be useful for a variety of uses, such as verifying the structure of holographically organized systems before fixing them in place. It also would be useful for interactively manipulating and inspecting three-dimensionally structured objects such as biological specimens. Integrating three-dimensional imaging with holographic trapping might seem straightforward because both techniques can make use of the same objective lens to collect and project laser light, respectively. However, conventional three-dimensional imaging methods, such as confocal microscopy, involve mechanically translating the focal plane through the sample. Holographic traps, however, are positioned relative to the focal plane, and would move as well. The trapping pattern would have to be translated to compensate for the microscope's mechanical motion, which would add substantial complexity, would greatly reduce imaging speed, and would likely disrupt the sample undergoing examination and analysis.
Optical methods are increasingly widely used to manipulate and track nanostructured materials. The high-numerical-aperture optics required for such studies offer optimal spatial resolution, but severely restrict the accessible depth of focus to within a few micrometers. Confocal and deconvolution microscopies overcome this limitation by scanning through the sample and assembling the resulting axial slices into a volumetric data set. Scanning takes time, however, and so is of limited utility for studying the dynamic processes that evolve in three dimensions. Some implementations also require the sample to be fluorescently labeled, which may not be desirable. Scanning probe microscopy and electron microscopy both have superior spatial resolution, but typically are not compatible with three-dimensional micromanipulation techniques, particularly under environmental conditions. Consequently, there exists a need to efficiently manipulate and track nanostructured materials, particularly rod-shaped materials without the severe limitations of conventional methods.